The Effect of High vs. Low Fluid Volume on Ocular Parameters in Prone Spine Surgery

In the prone position, the intraocular pressure and optic nerve sheath diameter increase progressively with time as compared with those in the supine position. Excessive fluid infusion may further increase intraocular pressure and optic nerve sheath diameter. Pulse pressure variation (PPV) is a dynamic index which can effectively assess fluid responsiveness during general anesthesia.Therefore,the investigators have designed a study to compare the effect of different fluid volume infusion guided by low and high PPV indices on intraocular pressure and optic nerve sheath of patients undergoing prone spine surgery with general anesthesia. One group of the patients will receive relatively loose fluid infusion (target value of PPV: 6%-9%),while the other group of the patients will receive limited fluid infusion (target value of PPV: 13%-16%).